"I humbly ask for your support to be Montana's senator, " Daines said at an event in his hometown of Bozeman.

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The freshman senator has long been expected to run, and sources told The Hill last week that he would announce Wednesday.

Daines begins the race as a slight favorite to succeed Baucus in the GOP-leaning state. National Democrats are hopeful Montana Lt. Gov. John Walsh (D) will run a strong race, though he got a primary challenger in former Montana Lt. Gov. John Bohlinger (D) on Wednesday.